1. Technical Field
The present invention relates to a magneto-optical recording medium, and a method of manufacturing the same.
2. Description of the Related Art
An ordinary magneto-optical recording medium, for example, a magneto-optical disk is composed as shown in a schematic sectional view in FIG. 6, in which a light-permeable first dielectric layer 2 of SiN and 80 nm to 100 nm in thickness, a magneto-optical recording layer 3 of, for example, TbFeCo, a light-permeable second dielectric layer 4 of SiN and 20 nm to 40 nm in thickness, and a metal reflecting film 5 of Al and 50 nm to 60 nm in thickness are sequentially laminated on, for example, a light-permeable substrate 1.
In the magneto-optical recording medium having the structure as shown in FIG. 6, the light L enters from the substrate 1 side.
The magneto-optical recording layer 3 comes to have light permeability as its thickness is defined less than 30 nm, for example, 15 nm to 25 nm, and by making use of multiple interference with the metal reflecting film 5, an enhancement effect of Kerr effect is obtained, and therefore while keeping constant the optical performance index, that is, Rxc2x7xcex8K (R: reflectivity, xcex8K: Kerr rotational angle), the reflectivity R can be lowered, and an optimum thermal design is realized by combination with the metal reflecting film.
First, its optical aspect is explained. If the performance index remains constant when the film composition is changed, the signal amplitude and disk noise keep a constant level, and the S/N is not changed. However, in a magneto-optical disk, shot noise is dominant in the high frequency region. Since the shot noise level is proportional to the square root of the quantity of detected light on the detector for detecting a signal, the noise level is lower as the reflectivity is lower. As far as thermal properties and servo characteristics permit, the S/N is raised in the design of lower reflectivity.
To lower the reflectivity, it may be realized in a structure in which a SiN dielectric layer and a thick magneto-optical recording layer are applied on a substrate. In this case, however, if the thickness of the SiN dielectric layer is changed, the reflectivity is not lower than 20%. To further lower the reflectivity, it is effective to form the magneto-optical recording layer in a thickness of less than 30 nm, and dispose the reflecting films sandwiching the dielectric layer. In particular, when the thickness of the magneto-optical recording layer is less than 20 nm, the light transmittance is raised, and by making use of the multiple interference with the reflecting film, the reflectivity can be varied in a wider range. As the reflecting film, generally, a high reflectivity metal mainly composed of Al is used.
Next, the thermal aspect is explained. From the thermal aspect, by properly selecting the thickness of the metal reflecting film and the film thickness of the second dielectric layer, the thermal properties are controlled. However, since the light spot size is made smaller in diameter due to the recent high density trend of magneto-optical disks, if the reproduction light intensity is raised, there occurs a problem that the recorded marks may be erased, for example, by repeated reproductions. In particular, in a short wavelength light source, since the light absorption is concentrated on the surface of the magneto-optical recording layer, the temperature of the magneto-optical recording layer rises before the heat reaches the metal reflecting film, and therefore in the conventional film composition, the thermal characteristics cannot be controlled.
Generally, when the film thickness is reduced in a metal film, the mean free path of electrons is smaller and heat conduction declines, and it is known that the heat conduction drops suddenly when the film thickness is smaller than about 30 nm. It is no exception in the magneto-optical recording layer of metal film such as TbFeCo, and therefore if attempted to maintain the optical characteristics by defining the film thickness at less than 30 nm, the problem is the temperature rise due to a drop in heat conduction.
That is, in the magneto-optical recording medium, the optical performance index and suppression of temperature rise are contradictory problems.
Therefore, relating to the magneto-optical recording medium, it is an object of the invention to present a magneto-optical recording medium capable of improving the thermal properties while maintaining the optical performance index, and a method of manufacturing the same.
The magneto-optical recording medium of the invention is formed by laminating a light-permeable first dielectric layer, a magneto-optical recording layer, a light-permeable metal layer of 1 nm to 10 nm in thickness and having larger heat diffusion as compared with the magneto-optical recording layer, a light-permeable second dielectric layer, and a metal reflecting layer, sequentially from the light incidence side.
The method of manufacturing a magneto-optical recording medium of the invention comprises steps of forming a light-permeable first dielectric layer, a magneto-optical recording layer, and a metal layer of 1 nm to 10 nm in thickness and having a larger heat diffusion as compared with the magneto-optical recording layer sequentially on a light-permeable base material, a step of smoothing the surface of the light-permeable metal layer having large heat diffusion compared with the magneto-optical recording layer after the film forming steps, and a step of forming a light-permeable second dielectric layer and metal reflecting layer sequentially afterwards, whereby the light enters the magneto-optical recording medium from the light-permeable base material side.
Also, the method of manufacturing a magneto-optical recording medium of the invention comprises a step of respectively forming a metal reflecting layer, a light-permeable second dielectric layer and a metal film having large heat diffusion compared with the magneto-optical recording layer sequentially on a base material, a step of smoothing the surface of the light-permeable metal layer after the film forming step, and a step of forming a magneto-optical recording layer and a light-permeable first dielectric layer sequentially afterwards. Thus, the light enters the magneto-optical recording medium from the opposite side of the light-permeable base material side.
In the invention, the transmission means a specific light-permeable characteristic of reproducing light radiated to a magneto-optical recording medium.
According to the magneto-optical recording medium of the invention, between the magneto-optical recording layer and the metal reflecting film, a metal layer is placed in contact with the magneto-optical recording layer by selecting the film thickness in a range of 1 nm to 10 nm so as to be light-permeable, and having a larger heat diffusion as compared with the magneto-optical recording layer, and therefore while maintaining the optical index, this metal layer is responsible for heat conduction of conduction electrons, and the effective heat diffusion in the magneto-optical recording layer is enhanced, so that the thermal properties may be enhanced.
According to the manufacturing method of the invention, which includes a surface treatment step of smoothing the surface of the magneto-optical recording layer, increase of noise due to worsening of the surface properties can be avoided.